Problem: In math class, the girl to boy ratio is $10$ to $8$. If there are a total of $90$ students, how many boys are there?
Answer: A ratio of $10$ girls to $8$ boys means that a set of $18$ students will have $10$ girls and $8$ boys. A class of $90$ students has $5$ sets of $18$ students. Because we know that there are $8$ boys in each set of $18$ students, the class must have $5$ groups of $8$ boys each. There is a total of $40$ boys in math class.